Brown Dwarf Planets and Habitability

byPaul GilsteronJune 21, 2010

Are planets common around brown dwarfs? We aren’t yet in a position to say, but the question is intriguing because some models suggest that the number of brown dwarfs is comparable to the number of low-mass main sequence stars. That would mean the objects — ‘failed’ stars whose masses are below the limit needed to sustain stable hydrogen fusion — could be as plentiful as the M-dwarfs that far outnumber any other type of star in the galaxy. If planets form around brown dwarfs, then we have to add them to our list of possible abodes for life.

Evidence for Brown Dwarf Planet Formation

But first, to the planet question. We can find suggestive analogs to planet formation around brown dwarfs in nearby space. The star Gl 876, some fifteen light years away, is not a brown dwarf, but this M-dwarf is only 1.24 percent as luminous as the Sun, with most of its energy being released at infrared wavelengths. We now know that at least three planets, two of them gas giants similar to Jupiter, orbit the star. Among brown dwarfs themselves, we have cases like 2M1207b, MOA-2007-BLG-192Lb and 2MASS J044144. In fact, the planet orbiting the second of these brown dwarfs is one of the smallest exoplanets known at 3.3 Earth masses.

As Andrey Andreeschchev and John Scalo (University of Texas) noted in a 2002 paper (thanks to Centauri Dreams regular ‘andy’ for the tip), we can extrapolate from what we find in our own Solar System to lower-mass stars, with simulations indicating that terrestrial-mass planets can form around low-mass objects like these as long as sufficient disk material is available. The authors study whether or not such planets can be habitable, noting this key fact about brown dwarf evolution: The brown dwarf is continually fading as it releases gravitational potential energy. As the object fades, its habitable zone moves past any worlds in it.

Time, Tides and Habitability

Is there time, then, for life to form on such a planet? When andy sent the pointer to this paper, he added an intriguing comment of his own:

It’d be interesting to come up with some scenarios for evolution on such a planet whose star decreases in luminosity as it ages (as opposed to more conventional stars that brighten as they age) – perhaps life might begin in the cloud layers of an initially Venus-like planet, moving to the surface as the atmosphere cools and the oceans rain out of the atmosphere, and finally moving to a more Europa-like state with the oceans frozen under an ice layer.

Now that’s a chewy science fiction scenario for the writers who frequent these pages to work on. Andreeshchev and Scalo note that a brown dwarf planet will be within the tidal lock radius, meaning the planet will always present one side to its star even when the brown dwarf is young, but we do have some studies showing that atmospheres can remain viable in such settings, so this may not rule out life. A bigger question is just how long the habitable zone will remain habitable and how, as andy notes, life might adapt. Clearly, evolutionary time-scales on a brown dwarf planet could be much different from those on Earth, but the paper notes that a habitability duration of less than 0.1 billion years would present real issues about the viability of complex life.

I can’t get Andreeshchev and Scalo’s diagram reproduced well enough to display well here, but they study the duration of residence in the evolving habitable zone as a function of the planet’s distance from the brown dwarf, assuming a circular orbit. They find that much depends on how we set limits on the habitable zone, but in general habitability durations of a billion years are possible for planets within 2-3 Roche radii for brown dwarfs above 0.03 solar masses. The Roche limit defines how close a planet can be to its host star before being torn apart by tidal forces. A habitable zone duration of up to 4 billion years is possible only close to the Roche limit, but could theoretically occur for brown dwarfs as small as 0.04 solar masses.

In fact, if you push these numbers to their upper limits, you can work out a habitable zone that has a duration of up to 10 billion years for a brown dwarf with a mass of 0.07 solar masses, as long as you’re willing to skirt the Roche limit about as close as possible. The authors are working, by the way, with a habitable zone definition that involves liquid water at the surface, the classic formulation of habitable zone rather than more recent extensions of the idea.

Temperature and Intelligence

This is a short but fascinating paper, and here’s something that catches the eye:

…if development of intelligence is partially driven by cooling episodes, as suggested by Schwartzman & Middendorf (2000), then on BD planets cognitive evolution may be expected to contain a stronger continuous component than on Earth.

I leave it to the science fiction writers to come up with depictions of the societies that may result. And I’ll end with the thought that if we do decide brown dwarf planets are not uncommon, and that complex life may find ways of evolving on such worlds, then nearby space may be littered with astrobiologically interesting destinations that are largely unknown to us. Or will be until infrared surveys like WISE tell us just how common brown dwarfs really are in our stellar neighborhood.

The paper is Andreeshchev and Scalo, “Habitability of Brown Dwarf Planets,” Bioastronomy 2002: Life Among the Stars. IAU Symposium, Vol. 213, 2004 (abstract).

Comments on this entry are closed.

IstvanJune 21, 2010, 10:33

The prospect of life evolving on a planet orbiting around a brown dwarf is indeed fascinating. It seems extremely clear however that the timescale of habitable-zone migration is so gradual as to be not relevant to the other reason habitable objects orbiting brown dwarfs are interesting, and that is the prospect of our own use of them.

With a brown dwarf within a parsec of our own system being a hopeful and still-possible scenario for a more-accessible long-term destination than the Alpha Centauri system, I’d say the idea and hope that there could be something even semi-habitable in close orbit around that brown dwarf is a lot more exciting than just rocks or iceballs.

Sorry, I had an afterthought to my enthusiasm: The magnetic and radiation environment surrounding Jupiter is highly unfavorable to the orbit of Callisto or thereabouts. I would assume that a brown dwarf object is liable to likewise have a similarly unpleasant radiation environment, unless the radiation environment around Jupiter has a large component based on interaction between Jupiter’s magnetic field and the solar wind.

I have read negligible articles in the literature that would have bearing on this, and am speculating wildly. Can anyone point me to relevant information?

It seems to me that the closest thing we currently have to a brown dwarf is Jupiter and its moons. Extrapolating from that, we can expect a dangerous radiation environment. However, I have also heard that a few meters of ice (or water) make a sufficient shield.
If the planets have a sufficient amount of water that should not be a problem. Given the lower temperatures it is in fact likely that they will be mostly ocean worlds. The radiation will actually be an useful source of energy/oxidants :http://www.astrobio.net/exclusive/3506/europa%E2%80%99s-churn-leads-to-oxygen-burn

Depending on the mass of the planets and their distance from the brown dwarf, we should get Io/Europa analogues or, if it has enough mass to hold onto an atmosphere, we could get something different : a world that thanks to tidal heating (and infrared radiation) keeps the surface water liquid.
Any surface and the first few meters of water would probably be deadly for radiation, but they would also produce a “rain” of oxidants : a substitute for light ?

Andy’s postulation reminds me of Asimov’s ‘The Deep’. In this story, an intelligent race moves deeper into its parent world as its dying sun did not heat it. Migration in via evolution is a different take on this… good thought, andy.

Are photosynthetic pathways as we know them going to work in a BD sky? And if not, how are you going to get free oxygen in the atmosphere? Can anyone point me to a recent paper? I suspect any life on these systems is going to be taking another route.

Having said that I’m hanging out for the WISE data to be crunched and for a bunch of really close BDs to emerge that can be scrutinized for transits and RVs. For me the find of the year has been UGPS 0722-05, if only because it hopefully points to a new class of nearby objects.

” I would assume that a brown dwarf object is liable to likewise have a similarly unpleasant radiation environment, unless the radiation environment around Jupiter has a large component based on interaction between Jupiter’s magnetic field and the solar wind.”

There’s also the question of the Galilean moons’ lack of significant intrinsic magnetic fields. Those worlds are low-density, substantially iceballs, lacking the metals that they’d need to generate a magnetic field of their own that could deflect the charged particles of the Jovian radiation belts.

Let’s say that the Galilean-equivalent planets of a brown dwarf would be significantly larger than their prototypes, having grown in proportion somewhat in proportion to their primary, and that they were at least as dense as any terrestrial planet never mind more massive. Could they generate magnetic fields that would protect their surfaces from the worst of the radiation?

Just to answer the worries about brown dwarf magnetospheres – Jupiter’s is so hostile because it traps high energy protons from the Sun, plus there’s some kind of poorly understood acceleration process that adds energy to them. A brown dwarf is more like a star, producing some kind of stellar wind emissions, though that’s probably much less energetic than the radiation belts of Jupiter. Will be very interesting to find out just how less.

A brown dwarf just 0.075 solar masses is an almost star. It actually can sustain fusion, but only for 10 billion years before it sputters out. A red dwarf star, at 0.08 solar masses, can sustain fusion for 10 trillion years, demonstrating how much more efficient its Main Sequence is than the ‘brief’ warmth of a high mass brown dwarf.

Over the next decade, and with a little luck we are going to discover through WISE and other Instruments what the admittedly incomplete data seems to be suggesting to us. The ~4.5 light years between Sol and Alpha Centauri is not two distinct zones, but a single common zone or continuim with multiple ” Ort Clouds” a couple of Brown Dwarfs and perhaps even a ~ .o8 Solar Masses Red Dwarf Star between Sol and the Alpha Centuari/Proxima Centauri system. If this is indeed the case it is going to open up all sorts of opportunities to discover, explore and perhaps even colonize depending on what we find out there. I strongly suspect that by 2020 the definitions for “Interstellar” and “Interplanetary” in relation to Sol and Alpha Centauri are going to be undergoing a major re-write as real Interstellar Exploration is increasingly viewed as what happens outside of the Common Zone/Bubble of Sol, Alpha Centuari,Proxima Centauri, and the other stars and Brown Dwarfs that are in between. The current thinking is that ~4.5 LY is allot of distance between Sol and Alpha Centuari. In reality it all depends on what exists between them especially in terms of Brown Dwarfs or even a very small Red Dwarf star. Is there substantial Interstellar Space and a void between Sol and Alpha Centauri or is this going to turn out to be a Lilly pad exercise with lots of interesting things in between. It may be the latter, in which case Manned and Unmanned trips to the “other side of the Common Zone” i.e to Alpha Centauri may seem to be far more feasible, and much more likely then they are today. If there is a Brown Dwarf or even a Red Dwarf star lurking between Sol and Alpha Centauri we may be able to travel out to about 5 LY’s by 2100 given how much easier this makes the trip in terms of refueling, sling shotting and perhaps even Hybrid Solar Sailing.

When astronomers loosely say a BD ‘between’ us and Alpha C they really don’t mean between in the sense of a cosmic steping stone. The probability is large that any BD, if there is one or so, lies in a different 3 dimensional direction that helps nada with geting to Alpha C. Think 3-D sphere.

For me the find of the year has been UGPS 0722-05, if only because it hopefully points to a new class of nearby objects.

You might like to know that today an update was posted on the arXiv about this object. The distance has been increased to 4.1 parsecs, and the paper has been withdrawn from Nature and a revised version submitted to MNRAS.

@Adam. Those borderline objects just below the threshold for sustained fusion are an interesting resource for “deep future” survival of intelligence.

In effect, they are an already stoked fire, just waiting for the addition of a little mass too light them up. Drop in a Jovian class planet that is surplus to your needs, and on demand you have a minimum mass red dwarf with a lifespan of 10 trillion or so years.

For deep future survival, brown dwarfs represent a large resource of unburnt fusion fuel, either for harvesting slow use, or to light up a new star.

How would tidal heating and decaying orbits affect the habitability of such planets? Presumably, such a planet could move into the habitable zone and stay in it if it’s orbit keeps decaying at the correct rate.

If Brown Dwarfs have a scaled up version of Jupiter’s radiation belts, we may find an abundance of places that are bi0compatible, in the sense of having oxygen atmospheres as a by-product of the radiation + ice that can be expected. If this is the case, we might also find Europan planets with an abundance of subsurface oxygenated water, which could bubble out to form habitable caves in the ice above a cold ocean.

Hi Colin W
Good point. Laughlin & Adams now classic paper & book on cosmic eschatology (A dying universe: the long-term fate and evolution of astrophysical objects &“The Five Ages of the Universe”) covers the formation of new red dwarfs by BD collisions in the long era after the regular stars have long since sputtered out. About once every 0.1 trillion years there should be a collision and the stars last ~10 trillion years, so about 100 will exist at a time, up until the Galaxy’s dynamic relaxation disperses ~90% of the stars into the Void. The claustrophobic darkness of that era, some 10 million trillion years from now, sends shivers down the spine.

It seems to me that a BD may or may not have a super-Jupiter sized magnetic field, depending on its composition and internal dynamics. If it does have a large magnetic field, not fed particles by any solar wind from a companion, or surrounding nebula, it could be relatively empty of particles, and have low radiation levels in spite of the field. Or there could be particles churned up and spit out by the BD itself. So I would expect a wide range of different radiation environments around BD’s.

On the other hand, all the BD’s detected so far have been at least – what figure did I see somewhere – about 350 F, hot as an oven. It takes a few billion years for an object that big to shed the heat that it got from formation. So the universe may not be old enough to contain any “ice giants.” At least I don’t know of any evolutionary process that could have produced one in 14 billion years.

Probably the only safe bet is that there are stranger things out there than we can imagine!

The idea of dropping a Jupiter-sized object into a borderline BD to get a RD is a very cool idea for an experiment – in fact, the Universe has probably done this many times. There are a couple of problems, especially if you want to live near this new RD. First, you are going to get a large explosion, possibly resulting in an object less massive than the BD you started with. Second, what you are left with will be very unstable. Even ordinary RD’s have a habit of flaring up violently and unexpectedly. It’s beginneing to look like all RD’s, even the old, seemingly quiet ones, will flare if you watch them long enough. And you don’t want to be anywhere near one of those when it flares!

I’m new to this site, so please pardon me if I’m covering old ground. Like the rest, I can’t wait for the WISE results!

Don’t be so sure about the explosion. It would take some time for fusion to actually begin, since the brown dwarf would still have some contraction to do before the core temperature would get hot enough, and then the heat has to escape the core.

Since lit stars can form at masses as low as ~0.04 solar mass, provided the metallicity is 3x that of today’s value, it may be possible to stellarize such objects provided we could find enough metals to dump in them. If we need 5% of the Brown Dwarf mass in metals, for a brown dwarf of 40 Jupiter masses, we’ll need 2 Jupiter masses of metals = 660 earths worth. Considering the brown dwarf might have 2/5 of the required level already, that’s a slightly more manageable 400 earths. There *might* be enough comets around…

I don’t see how we can say the Milky Way galaxy will remain “as is” for 800 billlion years when it is going to merge with Andromeda and form a giant elliptical galaxy in a mere couple of billion years?

Is that guaranteed? Or is it more likely that they’ll just skim each other, resulting in them both becoming distorted spirals connected by a long stream of stars?

Anyway, if they do merge, it might result in lit brown dwarfs forming quicker than they would have, due to the massive star forming epoch that would be set off driving up the metallicity. But once all that’s died down, the only lit stars will be red and infrared dwarfs.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to your fellow readers, I'm probably not going to run it.